Most anyone who has used a communication circuit, and in particular telephone communication circuits, has experienced the effect of echo. An echo is a delayed reproduction of the original sound eminating from the speaker or signal generating device. In communication circuits, echos occur when electrical signals encounter imperfectly matched impedance junctions and, as a result, a portion of the signal is reflected back to the talker. Because these reflected signals require a finite travel time, the reflected signal is heard as an echo rather than sidetone. As distances increase the reflected signal takes longer and longer to reach the talker and the echo becomes more and more annoying. For reasonably short distances, where the delay is in the tens of milliseconds, such as might be encountered for transcontinental telephone calls in the United States, control of the echo may be obtained by opening the return circuit path, thereby preventing the reflected signal from returning to the talker. Although this is somewhat annoying, because responses from the listener may be blocked, thus preventing an appearance of face to face communication, effective communication can be obtained and thus the slight inconviences introduced by this technique are usually tolerated.
With the introduction of satellite communications the delay times are increased from tens of milliseconds to hundreds of milliseconds and interruption of a circuit path does not provide an effective solution to the two-way voice communication problem. In an article entitled "A 12-Channel Digital Echo Canceller", Donald L. Duttweiler, IEEE Transactions on Communications, COM-26, No. 5, May 1978, pp 647-653, it is pointed out that there is a delay of approximately 270 milliseconds in each path of a four-wire synchronous satellite circuit. Thus, the round-trip, or echo-path, delay exceeds one-half second. Because of this long echo-path delay two problems are encountered. The first is that any interruption of the transmission paths, such as by use of an echo suppressor, would preclude effective two-way communication between the parties to the telephone conversation. Second, the effect of the echo delay would be intolerable because it would severely affect the speaker's ability to communicate. For these reasons echo cancellers were introduced in order to reduce the amplitude of the echo signal and to permit effective two-way communication over satellite circuits.
Echo Cancellers usually employ a transversal filter or a finite impulse response (FIR) filter. It is known that the complexity of the echo canceller is proportional to the number of taps used in the transversal filter and the sampling frequency. Even for the terrestrial circuits the number of taps for prior art devices may be quite high. For example, a 384 tap echo canceller is not uncommon. Such a device would need to perform 3,072,000 multiplications per second, i.e., 8 kHz.times.384, for a normal voice channel. Furthermore the residual echo after cancellation is also proportional to the number of taps, since a small misalignment in each tap coefficient would add to the error in generation of a replica of the echo signal.
What an echo canceller does is the following: it generates the replica of an echo from the information that passes through the receiving path and subtracts it from the echo to be cancelled which is being transmitted along the transmitting, or return, path. Such arrangements are exemplified by U.S. Pat. No. 3,499,999, "Closed Loop Adaptive Echo Canceller Using Generalized Filter Networks", M.M. Sondhi, Mar. 10, 1970 and U.S. Pat. No. 3,500,000, "Self-Adaptive Echo Canceller", J. L. Kelly, Jr., et al., Mar. 10, 1970. Both consider the operation of a closed loop echo cancellation system for use in two-way communication circuits. The Sondhi invention teaches the use of a generalized filter network which has as one input the voice signal on the incoming path to the four-wire to two-wire transformation junction and has, as a second input, an error signal which provides the feedback input necessary to correct the echo signal replica. This replica is subtracted from the outgoing signals thus reducing the echo amplitude. The residual echo is the differential which is the error signal. The Kelly et al. invention accomplishes a similar result but generates a replica of the echo by synthesizing a linear approximation to the echo transmission path by means of a transversal filter. The replica signal is subtracted from the return signal, and as with the Sondhi echo canceller an error signal (residual echo signal) is obtained. A closed loop error control system is employed and it is self adapting in that it automatically tracks variations in the echo path which may arise during a conversation, for example, as additional circuits are connected or disconnected. A somewhat different technique is disclosed in U.S. Pat. No. 4,232,400, "Echo Control System", S. Yamamoto et al., Nov. 4, 1980, in which regression coefficient calculators and predictors are used in the generation of an echo replica. To simplify the generation of the replica and thus improve echo cancellation, this invention effects a "whitening" characteristic to the input signal.